Measurement of the $��\to ��^+ ��^- ��^0$ Dalitz plot distribution

Using the production reactions $pd\to {}^3\mbox{He}\,��$ and $pp\to pp��$, the Dalitz plot distribution for the $��\to ��^+ ��^- ��^0$ decay is studied with the WASA detector at COSY, based on a combined data sample of $ (4.408\pm 0.042) \times 10^4$ events. The Dalitz plot density is parametrised by a product of the $P$-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables $Z$ and $��$. For the first time, a deviation from pure $P$-wave phase space is observed with a significance of $4.1��$. The deviation is parametrised by a linear term $1+2��Z$, with $��$ determined to be $+0.147\pm0.036$, consistent with the expectations of $��$-meson-type final-state interactions of the $P$-wave pion pairs.
8 pages, 7 figures